The present invention relates to a method for measuring the distance and speed of objects according to the preamble of the main claim. Such methods for measuring the distance and speed of objects are employed for example as part of an automatic cruise control system of a vehicle to detect vehicles traveling ahead. A system of this type is also referred to as an adaptive cruise control (ACC) system.
Various methods have become known for measuring a distance with the help of electromagnetic waves (radar). In the case of the system known as FMCW radar, electromagnetic waves whose frequency is modulated essentially in the shape of a ramp between two values are continuously transmitted. In the known methods of this type the received reflected waves are mixed with the simultaneously transmitted waves. During the change of frequency, i.e. during the ramp of the modulation signal, conclusions about the elapsed time and thus about the distance of the object can easily be drawn from the intermediate frequency obtained from simple mixing.
If the reflecting object is moving relative to the measuring location, the reflected signal undergoes a Doppler shift. This is utilized in a method known from German Patent Application 40 40 572 A1 for measuring the distance and speed by measuring the frequency difference during a rise and during a drop in the frequency of the transmitted waves and calculating the speeds from the differential between the frequency differences, and calculating the distance from the mean of the frequency differences. In one exemplary embodiment of this known method the wave reflected from an object or the intermediate frequency signal derived therefrom is evaluated.
From German Patent Application 42 42 700 A1 a method is known for measuring the distance and speed of objects which performs a spectral analysis of the intermediate frequency signal which arises from mixing the transmitted and received signals and calculates the distance and speed of at least one object from the frequency of spectral lines.
One method according to the present invention for measuring the distance and speed of objects is further developed from the related art by the fact that in a method for measuring the distance and speed of objects using electromagnetic waves with a motor vehicle radar system, electromagnetic waves being transmitted and simultaneously received, the transmitted electromagnetic waves being modulated in the shape of a ramp, at least the signals received during one rise and one drop of the frequency of the transmitted signal being mixed in each case with the transmitted signal, a number of intermediate frequency signals being formed, and the distance and speed of the object being calculated using the intermediate frequency signals, a weather condition in the vicinity of the motor vehicle and/or a disturbance in the motor vehicle radar system is identified.
Using this extension of a method known from the related art according to the present invention, information can be stated in a simple and reliable manner about the environmental conditions and the operating state of the motor vehicle radar system. In particular it is possible to differentiate between the weather conditions of dryness and bad weather, and soiling of a covering of the motor vehicle radar system can be identified. The characteristic intermediate frequency signals are characterized according to the present invention by the fact that the intermediate frequency signals have approximately the same magnitude with respect to each other.
The preferred extension of the method according to the present invention provides that on the basis of the detected weather condition and/or disturbance the present range is determined and/or present system performance of the motor vehicle radar system is checked. For this purpose, it is advantageous to have a characteristic map for determining the current range and/or the current system performance, containing at least characteristic intermediate frequencies, stored in a memory unit of the motor vehicle radar system. On the basis of the determination of system performance a speed recommendation is signaled advantageously to the driver of the motor vehicle, so that the driver can adjust the speed to the external environmental conditions.
Radar signals are attenuated only to an insignificant degree by rain, and snowfall leads to a loss of range which depends on the density of the snowfall and the form of snow. In both cases radar reflections are generated which are characteristic for the weather conditions and whose amplitude is a measure of the available radar range. Part of the emitted electromagnetic waves is scattered by the raindrops or snowflakes and returns to the receiver. There these scatter echoes are detected and interpreted according to the present invention. Rain and snow reflections exhibit in particular the characteristic properties that they appear at an extremely close distance directly in front of the vehicle and are measured with an approximation using a relative speed corresponding to the speed of the vehicle itself (nearly stationary objects).